1) Field of the Invention
The present invention relates to an optical scanning device as an optical writing unit of an image forming apparatus, and to an image forming apparatus such as a digital copying machine, a laser printer, a laser facsimile machine, a laser plotter, or the like having the optical scanning device.
2) Description of the Related Art
An optical scanning device which is widely known in association with a laser printer or the like is generally so designed that a light beam coming from an optical source is deflected by a deflecting unit such as light deflector, condensed on a surface to be scanned through a scanning and imaging optical system such as fθ lens to form an light spot on the surface to be scanned, and with this light spot optical scanning in the main scanning direction is conducted on the surface to be scanned. The substantial material forming the surface to be scanned is a photoconductive surface of a photoconductive medium realized by a photoconductive photoconductors or the like.
In these years, writing density of optical scanning device keeps on increasing, and 1,200 dpi (dots per inch), 1,600 dpi or even higher writing densities are intended to be realized. In realizing such high writing densities, stability of the light spot is an essential factor. When “change in diameter of light spot on the surface to be scanned depending on the height of the image” is small, the light spot is said to be stable. In order to achieve the stability, the scanning and imaging optical system which images the light flux deflected by the deflector onto the surface to be scanned in the form of a light spot should have high performance. One factor that changes the diameter of light spot is “field curvature by the scanning and imaging optical system” as is known in the art, and a number of scanning and imaging optical systems that satisfactory correct this field curvature have been proposed heretofore. It is also important that the optical magnification of the scanning and imaging optical system is constant regardless of the image height, in order to support multi beam configuration that stabilizes the light spot and realize high writing density.
In one example of an image forming apparatus that uses the above optical scanning device, a color image is produced in the following manner. Four photoconductors are arranged in the feeding direction of recording sheet; light fluxes of light beams emitted from a plurality of light sources corresponding to the respective photoconductors are deflected by a single deflecting unit for scanning; the photoconductors are exposed to light simultaneously by a plurality of scanning and imaging optical systems corresponding to the respective photoconductors to form latent images; the latent images are visualized at developers each uses different colors, such as yellow, magenta, cyan, black and like; and the resultant visible images are sequentially transferred and fixed onto the same recording sheet so that they overlap with each other.
The image forming apparatus in which two or more sets of optical scanning device and photoconductor are used to obtain two-color images, multicolor images and the like color images is known as “tandem-type image forming apparatus”.
As such a tandem type image forming apparatus, for example, the following systems (1) to (4) in which a single light deflector is commonly used by a plurality of photoconductive media have been proposed.
(1) Opposite scanning system wherein light fluxes enter from both sides of the light deflector, and scanning is conducted while separating the deflected light fluxes as disclosed in Japanese Patent Application Laid-Open Nos. 11-157128 and 9-127443.
(2) Scanning system wherein a plurality of light fluxes which are substantially parallel and distanced in the sub scanning direction enter a light deflector, and scanning is conducted by a plurality of scanning optical elements corresponding to the plurality of light fluxes arranged in the sub scanning direction as disclosed in Japanese Patent Application Laid-Open No. 9-54263.
(3) Scanning system wherein light fluxes enter from either side of the light deflector, and the scanning and imaging optical system is structured by three lens: scanning lenses L1 and L2 through which a plurality of light fluxes which travel toward different surfaces to be scanned and scanning lens L3 which is provided for each surface to be scanned as disclosed in Japanese Patent Application Laid-Open Nos. 2001-4948, 2001-10107, and 2001-33720.
In the manner as described above, by sharing the light deflector with a plurality of surfaces to be scanned and reducing the number of light deflector, it is possible to reduce the size and cost of the image forming apparatus.
As an optical scanning device that is applicable to the tandem type image forming apparatus proposed by the applicant, the one disclosed in Japanese Patent Application Laid-Open No. 2000-350110 can be recited.
It is a primary object of the present invention to desirably correct field curvature in the main and sub scanning directions to realize stability of the light spot while satisfactory keeping “conjugating function” and “speed equalizing function”, and to provide constant optical magnification with respect to the image height of the light spot.
In recent years, a special surface represented by aspheric surface is generally employed for an optical element of an optical scanning device in order to improve the scanning characteristics. “Optical elements formed of resin (plastic)” are often used because they can be easily processed into a special surface and are cost-effective. Particularly in the tandem type image forming apparatus as described above, since the number of optical elements to be used is large, significant cost reduction effect is achieved by using optical elements formed of resin.
When optical elements formed of resin are used in the optical scanning device, reflection preventing coating is not often applied on the surface of the lens on account of ease of production. The reflected light generated by surface reflection of lens or by reflection between lenses returns to the deflecting surface of the light deflector where it is deflected and scanned again, and then enters the surface to be scanned to become ghost (flare) light. In consideration of recent improvement in sensitivity of photoconductor, the existence of ghost is getting inevitable.
Furthermore, in the optical scanning device, when the internal temperature of the optical box is elevated by the deflector that generates great heat such as polygon mirror, the heat will not uniformly transmit due to the air flow created by rotation of the polygon mirror or the shape variation inside the optical box, so that the internal temperature of the optical box has temperature distribution. Also in the scanning lens, the temperature does not change uniformly because of difference in manner of heat transmission, difference in shape of lens (difference in installation area in the optical box) and the like, so that temperature difference occurs depending on the position within the scanning lens.
When optical elements formed of resin are used in such an optical scanning device, the shape largely changes with temperature change and characteristics of the optical elements formed of resin also change because resins have larger coefficient of thermal expansion than glass.
In the tandem type image forming apparatus, the light fluxes directed to the respective photoconductors pass thorough different scanning lenses. Since different temperature distributions occur in the different scanning lenses due to the temperature distribution within the optical box accommodating the scanning lenses, change in shape and change in refractive index of the scanning lens is not uniform, change amount of scanning line length and change in speed uniformity differ among the different photoconductors. After visualizing these latent images by developers using different colors of developing agents of yellow, magenta, cyan, black and the like, the resultant visible images are transferred to the same recording sheet in overlapped manner and fixed, whereby a color image is obtained. In this case, so-called “out of color registration” occurs. In particular, when the scanning lens nearest to the deflector that generates great heat within the optical box such as polygon mirror, the change in optical characteristics is also large.
In the case of continuous output, in particular, when a large number of sheets are continuously output, the internal temperature (temperature in the optical box) rises due to heat generation by the deflector. This results in change in temperature distribution in each scanning lens, occurrence of out of color registration described above, as well as change in variation amount thereof. As a result of this, color difference occurs between the first outputted image and the last outputted image due to out of color registration.
For solving the above problem of “change in scanning line length”, a method is known wherein light receivers are provided on the write starting side and the write ending side, respectively, and the image frequency of each light beam is adjusted according to the difference in light receiving time between these light receivers as disclose in Japanese Patent Application Laid-Open No. 9-58053. For employing this method to the tandem type image forming apparatus wherein “light deflector is shared by a plurality of surfaces to be scanned”, space for installing the light receiver is required on the write ending side, so that it becomes difficult to keep the available writing width.
Furthermore, in the method wherein light receivers are provided on the write starting side and the write ending side, respectively, and the image frequency of each light beam is adjusted according to the difference in light receiving time between these light receivers, the length of scanning line at each photoconductor can be corrected, however, the change in speed uniformity due to temperature distribution of each scanning lens can not be corrected. For this reason, even if dot positions in the main scanning direction are corrected at starting and ending points of writing at each photoconductor, dot positions at the middle points in the main scanning direction do not coincide, so that out of color registration occurs.
In the tandem type optical scanning device, there are many cases that the scanning lens nearest to the deflector that generates great heat such as polygon mirror is formed of glass for solving the above problem. However, the scanning lens formed of glass entails the problem of significantly larger cost compared to the scanning lens formed of resin.
Furthermore, the tandem type image forming apparatuses as described in (1) to (3) above entail the following problems.
With the system (1) exclusively, only two different surfaces to be scanned can be scanned, and four color writing cannot be conducted. Furthermore, since light beams directed toward the respective surfaces to be scanned pass through the different scanning optical elements, the beam position deviates from each other between the respective surfaces to be scanned, and the “out of color registration” is likely to occur.
In the system (2), the size of apparatus such as light deflector increases. This also leads the problems of accompanying noise, increase in consumption power, impaired durability, and raise in cost due to increased number of optical elements. Furthermore, since light beams directed toward the respective surfaces to be scanned pass through the different scanning optical elements, the beam position deviates from each other between the respective surfaces to be scanned, and the “out of color registration” is likely to occur.
In the system (3), since the scanning lens L1 has positive power in the sub scanning direction, and the light fluxes directed toward the respective surfaces to be scanned are converged, it is difficult to separate the light fluxes to the respective surfaces to be scanned.
Furthermore, for solving the problem regarding the ghost light, inventions disclosed in Japanese Patent Application Laid-Open Nos. 7-287180 and 7-230051 can be exemplified. However, since they are provided with shift or tilt of scanning lens or light shielding member inside the apparatus, they still have the problems of deterioration of optical characteristics and complication of the apparatus. Also, neither of these inventions has solve the problem that the light reflected at the lens surface of the scanning lens or reflected between lens surfaces returns to the light deflector and becomes ghost light on the surface to be scanned.